1. Field of the Invention
This invention, in general, relates to non-destructive testing methods and apparatus for determining dimensional differences between objects having complementary shapes and, in particular, to interferometric methods and apparatus by which a hologram is utilized to determine dimensional differences between precision optical surfaces and molds which are used in their fabrication.
2. Description of the Prior Art
Holography, or wavefront reconstruction, is a process by which the amplitude and phase variation across a wavefront from an object can be recorded photographically and subsequently reproduced. The photographic record of the wavefront from the object is referred to as a hologram and the wavefront reproduced from the hologram is said to have been reconstructed. The reconstructed wavefront is identical in form to the original wavefront from the object and produces the same effects as would the original.
One important practical application of wavefront reconstruction has been its utilization in the area of interferometry, a non-destructive test method which utilizes light interference phenomena for measuring geometric differences between objects. The fact that a wavefront reconstructed from a hologram can substitute for the original wavefront from an object has lead to several interesting techniques in this area. One is referred to as a double exposure technique wherein a hologram of an object is exposed and then reexposed with the object distorted. The reconstructed hologram results in a fringe pattern indicative of the displacements suffered by the object.
Other techniques interfere a reconstructed wavefront from a hologram of an object with an actual wavefront from the object to develop an interference pattern which indicates any changes (which may have occurred) in the object between the time its holograph was constructed and a later time.
It is also well-known to utilize holographic interferometry to compare wavefronts from parts from a manufacturing process with a holographic reconstruction which generates a wavefront representative of a standard for that part. Here, the holograph is usually generated by a computer or from a master or "perfect" part. Both methods work well, but each has its own particular shortcomings. The computer technique is expensive and time consuming, and the generation of a hologram from a master or "perfect" part requires that such a part exist, a requirement that can rarely be satisfied particularly during the early stages of manufacturing.
It is, therefore, a primary object of the present invention to provide an improved holographic interferometric method and apparatus by which parts from a manufacturing process can be easily measured in a non-destructive manner to determine their acceptability.
It is another object of the present invention to provide methods and apparatus by which a hologram can be utilized to determine dimensional differences between objects having complementary shapes.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. The invention accordingly comprises the apparatus and method possessing the sequence of steps which are exemplified in the following detailed disclosure.